1. Field of the Invention
The invention relates generally to the field of electric motors, and in particular to a sensor and method for accurately sensing a position of a rotor in a brushless electric motor using a magnetic sense element and linear output Hall effect sensors.
2. Description of Related Art
Electric motors that require controlled armature current waveforms (in order to rotate smoothly, for example) also require accurate rotor position sensing. Some motors use sensorless technologies, but these technologies do not provide accurate rotor position sensing at very low speeds and are not smooth upon startup of the motor. Other motors inherently cannot use sensorless technologies and must incorporate a rotor position sensing mechanism. Currently, state of the art motors use either an encoder or a resolver together with associated electronic circuitry to determine rotor positions. Depending on the resolution required, however, these solutions can become prohibitively expensive within applications that require low cost motors.
In particular, many electric motor applications require smooth rotation and/or accurate control. Brushless motors typically achieve this by using 3-phase sine-wave commutation and accurate rotor position detectors, usually in the form of an encoder or a resolver. The accurate rotor position detector ensures that the sine wave remains synchronized with the rotor, thus avoiding commutation-induced torque ripple. Methods presently used in the industry for accurately detecting rotor positions use encoders and resolvers and have been known and employed in motor drives for many years.
Encoders sense mechanical motion, and translate the sensed motion into electrical signals. Optical encoders are the most common type of encoder. An optical encoder typically includes a housing to support precision bearings and electronics, a shaft with a disc that is called an xe2x80x9coptical discxe2x80x9d and has alternating clear and opaque segments, a light emitting diode (LED), and a photo transistor receiver. A beam of light produced by the LED is aimed at the optical disc. When the optical disc rotates, the light beam passes through the clear segments but is blocked by the opaque segments so that the optical disc effectively pulses the light beam. The pulsed light beam is received by the photo transistor receiver. The photo transistor receiver and the circuitry inside the encoder together provide signals to a motor controller outside the encoder and can also perform functions such as improving noise immunity. Encoders in their simplest form have one output to determine shaft rotational speed or to measure a number of shaft revolutions. Other encoders have two outputs and can provide direction-of-rotation information as well as speed and number of revolutions. Still other encoders provide an index pulse, once per revolution, which indicates an absolute rotor position. The description thus far relates specifically to incremental encoders, where upon startup, the position of the encoder is not known. A second type of encoder, called an absolute encoder, has a unique value for each mechanical position throughout a rotation. This unit typically consists of the incremental encoder described above with the addition of another signal channel that serves to generate absolute position information, typically of lesser accuracy. Within an absolute encoder that is provided with an index pulse, the accuracy improves once the rotor traverses the index pulse. Incremental encoders can be acceptable within asynchronous motors, where speed feedback is most important. Absolute encoders are desirable within synchronous motor applications, where both position and speed feedback are important.
Another class of high resolution encoders is produced by several companies, and is referred to as xe2x80x9csine/cosine encodersxe2x80x9d. Sine/cosine encoders generate sine and cosine signals rather than pulse waveforms. When used with additional electronics, processor capability and software, sine/cosine encoders indicate rotor position with fine resolution.
Encoders of all types are precision built, sensitive devices that must be mechanically, electrically and optically matched and calibrated.
Resolvers, on the other hand, typically provide one signal period per revolution and are known to be highly tolerant of vibration and high temperatures. A typical use of this technology would include a resolver generating two signals, both a sine-wave signal and a cosine-wave signal, for each revolution. An advantage of using resolvers is that they provide absolute rotor position information, rather than incremental information as is typical with most encoders. A primary drawback, however, is that resolvers deliver increasingly poor performance at low speeds. Because of this limitation, the speed control range possible with resolvers is much smaller than with encoders, on the order of 200:1. Accordingly, use of resolvers is typically limited to applications that do not require high quality motor control over a wide speed range. As with encoders, resolvers are precision built, commercially available sensing devices that can be fragile and expensive.
Ring magnets and digital Hall effect sensors are often used as a rotor position sensing mechanism within brushless direct current (DC) motor applications where square-wave or six-step drive is used. This method of sensing provides low resolution, typically six position steps per electrical cycle when using three sensors. Six-step drive does not require high resolution rotor position sensing, however, so this is acceptable. At the same time, these drive methods do not result in ripple-free torque from the motor either. This may be unacceptable in a variety of applications.
Accordingly, a need exists for an accurate, low-cost device that senses rotor position and detects rotational speed. According to an embodiment of the invention, this need is satisfied by providing an assembly that includes a magnetic sense element such as an inexpensive sense ring magnet and two analog Hall effect sensors. In this embodiment the sense element is fixed with respect to the motor rotor, and the sensors are fixed with respect to the motor stator.
The sense ring is magnetized in an alternating north-south fashion with a number of poles that corresponds to a number of motor field poles. The Hall effect sensors are placed so that they measure the magnetic flux tangential to, and at some distance from, an outer circumference of the ring.
Orienting the Hall effect sensors to measure magnetic flux tangential to an outer circumference of the ring and at some distance from the ring results in a Hall effect sensor output voltage waveform that is substantially triangular, with a highly linear portion centered at zero flux, between the minimum and maximum peaks. This linear portion can be decoded, e.g., using an analog-to-digital (A/D) converter and control software, into an accurate measure of rotor position. The cycle or output waveform repeats itself for every pole pair. For example, where there are two evenly spaced pole pairs, the output waveform of a Hall effect sensor will repeat twice for each mechanical revolution, i.e., will have two complete electrical cycles. Accordingly, the inventive method can be used to decode rotor position within or relative to a complete electrical cycle, but not necessarily within a complete mechanical rotation that includes more than one electrical cycle, unless an absolute position reference such as an index pulse is also provided.
The relationship between electrical and mechanical degrees is given as xc2x0 E=xc2x0 Mxc2x7PP, where xc2x0 E represents electrical degrees, xc2x0 M represents mechanical degrees, and PP represents the number of magnetic pole pairs of the motor. By detecting absolute rotor position within a complete electrical cycle, current can be controlled accurately at all times to result in smooth rotation of the rotor.
According to another embodiment of the invention, the two Hall effect sensors can be placed further away from the sense ring, so that each Hall effect sensor outputs a substantially sinusoidal waveform. When the two Hall effect sensors are placed 90 electrical degrees apart, one output becomes a sine wave and the other becomes a cosine wave.
Additional features and advantages of the invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings. The accompanying drawings illustrate, by way of example, the principles of the invention.